// 画弧线
settings.outformat = "pdf";
size(3cm, 0);
draw((-1.2, 0)--(1.2, 0));
draw((0, -1.2)--(0, 1.2));

/* arc 从 -90 to 0 为逆时针.
等价于从画弧线从 270到360. */
path p2 = arc((0, 0), r = 1, angle1 = -90, angle2 = 0);
path p2b = shift(2.0, 0) * p2;
draw(p2b, arrow = Arrow(TeXHead));